1. Field of the Invention
The present invention relates to a light beam deflector for deflecting a guided light beam in an optical waveguide by generating a surface elastic wave in the optical waveguide and diffracting the guided light beam with the surface elastic wave, and more particularly to a light beam deflector for deflecting a light beam through a wide angle by combining guided light beams which are deflected by surface elastic waves on an optical waveguide, and a light beam deflector for simultaneously deflecting two light beams to record or read two images, respectively, at the same time.
2. Description of the Prior Art
There is known light beam deflector, as disclosed in Japanese Unexamined Patent Publication No. 61(1986)-183626, which comprises an optical waveguide made of a material capable of propagating a surface elastic wave therethrough. A light beam is applied to the optical waveguide and propagated therethrough as a guided wave. A surface elastic wave is produced in the optical waveguide and directed across the guided wave to diffract the guided wave through Bragg diffraction. The angle of diffraction (i.e., the angle of deflection) of the guided wave is continuously varied by continuously varying the frequency of the surface elastic wave. A light beam deflector of this type is advantageous over a mechanical light beam deflector such as a galvanometer mirror or a polygon mirror, an electro-optic deflector (EOD), and an acousto-optic deflector (AOD), since it may be small in size, light in weight, and highly reliable as it has no mechanical movable parts.
However, a light beam deflector employing an optical waveguide has a problem in that it cannot have a large angle of deflection. More specifically, since the light beam deflecting angle is substantially proportional to the frequency of the surface elastic wave, if a large angle of deflection is to be obtained, then the frequency of the surface elastic wave must be increased to a very high value. Therefore, the frequency of the surface elastic wave would have to be varied over a wide range. In addition, to meet the conditions for Bragg diffraction, the direction of travel of the surface elastic wave would have to be continuously steered to control the angle of incidence of the guided wave on the surface elastic wave.
To meet the above requirements, there has been proposed a light beam deflector, as also disclosed in Japanese Unexamined Patent Publication No. 61(1986)-183626, which has a plurality of interdigital transducers (IDT) for generating respective surface elastic waves, having frequencies which vary over different ranges. The IDTs are oriented such that they emit the surface elastic waves in different directions, and they are alternately switched into and out of operation.
This light beam deflector has a problem in that, since the diffraction efficiency is lowered around the crossover frequency of the surface elastic waves generated by the IDTs, the intensity of the deflected light beam varies depending on the angle of deflection.
An IDT which deflects the light beam through a large angle must be arranged so as to be capable of producing a surface elastic wave having a very high frequency. This will be described below with reference to an example. Assuming that the angle of incidence of the guided wave on the surface elastic wave is 8, then a, the angle of deflection of the guided wave due to an acousto-optic interaction between the surface elastic wave and the guided wave, is .alpha.=2.theta., which is expressed as follows: ##EQU1## where .lambda. is the wavelength of the guided wave, Ne is the effective refractive index of the optical waveguide with respect to the guided wave, and .LAMBDA., f, and v are the wavelength, frequency, and speed, respectively, of the surface elastic wave. Therefore, the deflection angle range .DELTA.(2.theta.) becomes: EQU .DELTA.(2.theta.)=.DELTA.f.multidot..lambda./Ne.multidot.v
If a deflection angle range .DELTA.(2.theta.)=10.degree. is to be obtained with .lambda.=0.78 .mu.m, Ne=2.2, and v=3500 m/s, for example, then the frequency range of the surface elastic wave, i.e., the range of high frequencies to be applied to the IDT, must vary by 1.72 GHz. If this frequency range is selected to be 1 octave so as not to be affected by secondary diffracted light, then the central frequency is f.sub.0 =2.57 GHz, and the maximum frequency is f.sub.2 =3.43 GHz. The wavelength .LAMBDA. of the surface elastic wave produced by the IDT to obtain the maximum frequency f.sub.2 becomes A=1.02 .mu.m, and the line width W of the electrode fingers of the IDT becomes W=.LAMBDA./4=0.255 .mu.m.
With the conventional photolithographic and electron beam printing processes used for fabricating IDTs, the limits for the line widths at present are 0.8 .mu.m and 0.5 .mu.m, respectively. It is therefore impossible to fabricate an IDT having very small line widths as described above. Even if such a finely fabricated IDT could be produced in the future, it would be difficult and highly expensive to produce a driver for generating frequencies as high as 3.43 GHz, and it would be difficult to apply a high voltage to such an IDT. Moreover, if the frequency of the surface elastic wave is increased, as described above, the wavelength thereof is reduced, and hence the surface elastic wave is absorbed to a greater extent by the optical waveguide, resulting in a reduction in the diffraction efficiency.
The IEEE Transactions on Circuits and Systems, vol. CAS-26, No. 12, p. 1072 [Guided - Wave, Acousto-optic Bragg Modulators for Wide-Band Integrated Optic Communications and Signal Processing] by C. S. TSAI, does not disclose a light beam deflector, in which a plurality of IDTs are switched into and out of operation, but does disclose a single IDT constructed as an IDT having arcuate electrode fingers, each having a continuously varying line width for continuously varying the frequency and the direction of travel of a surface elastic wave over a wide range. The disclosed arrangement eliminates the aforesaid problem of variation in the intensity of a light beam, which variation depends on the angle of deflection of the light beam, but still requires the surface elastic wave to have a high frequency.
There is a demand in the medical field, for example, for recording or reproducing two images, formed with different magnification ratios or under different image processing conditions, on a single medium for medical diagnosis. Light beam deflectors of the type described above may be employed for scanning a photosensitive medium with a light beam both to record an image and also to record two images simultaneously thereon. Two images may be recorded on the photosensitive medium by dividing the light beam deflection angle range into two deflection angle ranges and modulating the light beams in the respective deflection angle ranges with different image signals.
It is also possible to employ the above light beam deflectors in constructing a light beam scanning reading apparatus used for simultaneously reading out two images. In such a light beam scanning reading apparatus, an original medium with two images recorded thereon is scanned by a light beam to cause it to emit or reflect light. Then, an image signal is produced by photoelectrically detecting the light emitted or reflected when the original medium was exposed, dividing the deflection angle into two ranges, and thus extracting two image signals representing the two images, respectively.
Inasmuch as it is difficult for light beam deflectors of the above type to provide a large deflection angle range, however, any light beam deflection angle range available when using the light beam deflector for recording or reading out two images is small because it is a division of an already inherently small deflection angle range. With such a small deflection angle range, only small-sized images can be recorded or read out.